The proposed studies are designed to obtain preliminary data concerning the actions of estrogen on the development of progenitor cells of the adult rat hippocampal dentate gyrus. 17beta-estradiol reportedly enhances granule cell neurogenesis (the formation of new neurons) in the adult female rat hippocampal dentate gyrus. The cellular mechanisms and estrogen receptor(s) mediating this hormonal action are not known. We have shown that 17beta-estradiol and its transcriptionally inactive, natural stereoisomer 17alpha- estradiol elicit rapid and transient activation of the mitogen-activated protein kinase and phosphatidylinositol 3'-kinase-Akt signaling pathways in postnatal rodent neocortical cultures and neural progenitor cells of the adult rat hippocampal dentate gyrus. These responses appear to be mediated by a putative estrogen receptor (ER), "ER-X", which is distinct from the intranuclear estrogen receptors ER-alpha and ER-beta. "ER-X" is plasma membrane associated and developmentally regulated and returns following ischemic brain injury. Progenitor cells of the adult rat hippocampal gyrus are enriched in "ER-X" protein and lack ER-alpha and ER-beta mRNA and protein. We propose a series of integrated and complimentary cell biological and biochemical analyses to compare the actions of 17alpha and 17beta-estradiol on the development of progenitor cells. WE hypothesize that "ER-X" and 17alpha-estradiol, its preferred ligand, play an important role in the development and remodeling of the hippocampus. We will test this hypothesis by focusing on the effectiveness of 17alpha estradiol in comparison with 17beta-estradiol with respect to (i) neurogenesis, (ii) neuronal differentiation, and (iii) the intracellular signaling pathways that mediate these actions. The expression of "ER-X" in the adult hippocampal dentate gyrus suggests that its developmental role could be reactivated to enhance functional hippocampal remodeling in the course of disorders of cognition, mood, and affect, such as major depression and bipolar disorder, conditions that may be associated with estrogen, since they are more frequent in females. Unlike 17beta-estradiol, 17alpha-estradiol has little affinity for ER-alpha or ER-beta and may be more effective and therapeutically safer. The results of these experiments will enable drug development and treatment of children and adults of both sexes without fear of undesirable effects mediated by ER-alpha or ER-beta.